The present invention relates to an improvement of a mass flow control system.
A mass flow controller is conventionally constructed as shown in FIG. 1. That is, a gas flowing through a gas inlet pipe 1 is discharged through a mass flow controller 14 into a gas outlet pipe 2 as a flow of the desired mass. The gas inlet pipe 1 has a sensor heater 3 wound thereabout, which in conjunction with a resistance in a sensor circuit 4, forms a bridge circuit. When the resistance of the sensor heater 3 is changed by the heat removing function of the gas flowing through the gas inlet pipe 1, to thereby unbalance the bridge circuit, the voltage of the bridge circuit is derived from the sensor circuit 4 and sent to a flow indicator 13 through a signal line 7.
On the other hand, a signal concerning the flow which is preset by a flow setter 12 is sent through a signal line 8 to a comparator 6, by which it is compared with an actual flow signal provided by the sensor circuit 4, and an output corresponding to the result of the comparison is provided to a valve operating unit 5. In response to the output from the comparator 6, the valve operating unit 5 controls the valve opening so that the gas of the desired mass flows through the valve. The mass flow controller 14 is supplied with electric power from the power source 11 through a power line 9.
However, in the conventional mass flow controller in which signals are sent and received through a cable 10, the signals are easily affected by electromagnetic noise. Particularly in the signal lines, since the signals are transmitted at a voltage as low as 0-5 volts, they are readily disturbed by the noise which causes the actual flow to be indicated erroneously, or sometimes noise becomes mixed in the flow signal and prevents the flow control from being performed properly. In the case where an error arises in the flow to be measured as a result of the advanced age of the sensor, data according to an erroneous signal is indicated as it is and, when it is necessary to input the data to a computer, the erroneous data is provided to the computer. In the case where a plurality of kinds of gases are to be used alternately, the fact that the sensitivity of the sensor varies with the kind of the gas necessitates that the gain of the sensor circuit 4 or the sensitivity of the indicator be changed each time the gas is changed. Further, in the case where a plurality of mass flow controllers are used to construct a system in which they are controlled by a single host computer, the thickness and weight of the cable increases in proportion to the number of the mass flow controllers used, thereby hindering the movement of the operator or requiring additional space to acommodate the cable.